This invention relates to an improved cadmium-free silver-copper-zinc based brazing alloy.
The element cadmium is added to the silver-copper-zinc ternary system to depress the melting point and thereby provide low melting brazing alloys for brazing most ferrous and nonferrous metals. These alloys are well described in prior art and listed in the BAg series of the American Welding Society (AWS) classification. The cadmium containing alloys are in the lower temperature brazing range of the BAg series. These cannot be used in food handling equipment because of the toxic nature of cadmium. The only low temperature cadmium-free brazing alloy in the AWS classification is an alloy in the silver-copper-zinc-tin system where tin has been added to the Ag-Cu-Zn system to depress the melting point. The Ag-Cu-Zn-Sn system is also described in U.S. Pat. No. 3,862,838. In addition, U.S. Pat. No. 4,052,531 describes an indium containing silver-copper-zinc brazing alloy which has a brazing temperature range comparable to that of the cadmium containing silver brazing alloys.
The tin containing silver-copper-zinc brazing alloy in the AWS classification and the indium containing alloy described in U.S. Pat. No. 4,052,531 both have narrow melting ranges and are suitable replacements for BAgl and BAgla alloys in the AWS classification. The Bag2 and BAg2a alloys in the AWS classification have wider melting ranges suitable for brazing joints with wide gaps and a high degree of mismatch because the solid portion of the molten braze material acts to thicken it and slow its tendency to run out of the wide joint before solidifying. The wider melting range facilitates control of the brazing process. There are no alloys available as a suitable replacement of these two cadmium containing alloys.
The alloy of this invention has a low liquidus temperature and a wide melting range and allows for brazing of joints with wide gaps and a large degree of mismatch and is therefore suitable as a replacement for the cadmium containing BAg2 and BAg2a alloys in the AWS classification.
The alloy of this invention can be used for low temperature brazing of stainless steel used in food handling equipment where cadmium containing alloys cannot be used. Another advantage is that it matches stainless steel in color.